GCE 'O' 2013 Science (Chemistry) Paper 5

GCE 'O' 2013 Science (Chemistry) Paper 5

  8:00 AM    Unknown

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You are provided with two labelled solid samples of salt X, one in large stoppered hard-glass boiling tube and another in an open test-tube. You are also provided with a labelled sample of acid Y.

Carry out the following experiments. Carefully record your observations.

You should test any gases evolved.

The volumes given below are approximate and should be estimated rather than measured.

	test	observations
(a)	Place about 3 cm3 of limewater in a clean test-tube. Remove the stopper from the hard-glass boiling tube labelled salt X. Heat this boiling tube gently and then more strongly until no further changes are seen. You may see some condensation forming inside the boiling tube initially.	The colourless gas gives a white precipitate in limewater. [1] Gas evolved is carbon dioxide. [1]
Allow the boiling tube in part (a) to cool for use in part (c). Perform part (b) while you are waiting.
(b)	Soak the end of a wooden splint with water. Dip the wet end of the splint into the second sample of salt X (in the open test-tube). Make sure than the end of the splint is well covered with salt X. Adjust the Bunsen burner to give a blue flame. Place the wet end of the splint in the flame.	A green flame is observed. [1]

(c)	Add the contents of the large test-tube labelled acid Y to the cooled hard-glass boiling tube from part (a). Thoroughly mix the contents and transfer them to a 100 cm3 beaker. Gently heat and stir the mixture until no further change is seen. Do not allow it to boil. The liquid in the beaker is solution Z.	Black solid dissolves in acid Y to form a blue solution. [1]
(d)	Place about 2 cm3 of solution Z in a clean test-tube. Add 3 pieces of magnesium ribbon to the test-tube. You will need to observe this test-tube for a few minutes.	Reddish-brown/ pink solid formed over the grey magnesium ribbon. [1] The blue solution turns pale and finally colourless. [1] The test-tube feels warm. [1]
(e)	Place about 1 cm3 of solution Z in a clean test-tube. Add 1 cm3 of dilute nitric acid, followed by 1 cm3 of barium nitrate solution.	A white precipitate is formed in the blue solution. [1]
(f)	Place about 1 cm3 of solution Z in a clean test-tube. Add dilute aqueous ammonia slowly, with shaking, until no further change is seen.	A light blue precipitate is formed. [1] The light blue precipitate dissolves in excess aqueous ammonia giving a dark blue solution. [1]

(g)		Place about 2 cm3 of solution Z in a clean test-tube. Add one spatula-load of solid sodium chloride. Insert a rubber bung and shake the test-tube thoroughly to dissolve the solid.					The blue solution turns green. [1]
		Add water until the test-tube is over half full. Mix the contents of the test-tube thoroughly.					On adding water, the green solution turns back to blue. [1]
(h)	(i)		Suggest an identity for acid Y. Give evidence to support your suggestion.
			acid Y is	sulfuric acid
			evidence	in test (e), a white precipitate of barium sulfate is formed. This indicates the presence of sulfate ions. [1]
	(ii)		Consider the results for the experiments you have performed. Give two further conclusions about the ions and gases involved in these reactions. Give evidence to support each of your conclusions.
			conclusion 1			Salt X contains copper(II), Cu2+ ions.
			evidence	In test (f), a light blue precipitate which is soluble in excess aqueous ammonia to give a dark blue solution. This indicates the presence of copper(II) ions. [1]
			conclusion 2			Salt X contains carbonate, CO32-, ions
			evidence		In test (a), upon heating, salt X decomposes to give out carbon dioxide gas which gives a white precipitate in limewater. This indicates the presence of carbonate. [1]